1. Field of the Invention
This invention relates to photopolymerizable elements useful in the photomechanical arts, e.g., lithographic printing plates and printed circuits.
2. Description of the Prior Art
Alles, U.S. Pat. No. 3,458,311, July 29, 1969, discloses a homogeneous solid protective layer transparent to actinic radiation on a photopolymerizable stratum coated on a support. The protective stratum is nonstrippable as an unsupported film, substantially impermeable to oxygen, water-permeable and composed of a macromolecular polymer or polymer mixtures soluble in water or water mixed with a water-miscible organic solvent. The layer imparts enhanced speed and contrast to photopolymerizable strata because of oxygen impermeability. The layer is more easily applied and removable during the development washout of the exposed photopolymerizable stratum than the unitary cover sheets of the prior art. However, the protective layers of the Alles patent, when using the macromolecular polymers alone or in admixture, tend either to have poor adhesion for the photopolymerizable layer or under conditions of high humidity, become tacky and tend to be damaged by material in contact with its outer surface. The previously proposed protective layers tend to adhere to image films, i.e., photographic negatives, "goldenrod" paper and opaquing material conventionally used on the negatives when such materials are placed in contact with said layers during exposure in vacuum printing frames, particularly under conditions of high humidity. The adhesion usually causes the protective layer to pull away from the photopolymerizable layer when the image film is removed after exposure. Detachment of the protective layer before and during exposure is detrimental to the quality of the photopolymer layer, especially in the conventional step-and-repeat exposure machines used in making lithographic printing plates. The damage caused by the detachment of the protective layer manifests itself by lowering the photographic speed and causing poor image quality because of reciprocity failure under conditions of low intensity exposure and insufficient vacuum in the printing frame. The resulting printing plates exhibit poor wear qualities and also poor exposure and processing latitude.
Several U.S. patents are known wherein photosensitive materials have layers which contain solid particles. The solid particles are not present in a layer for the purpose of improving adhesion between the particle-containing layer and another layer of the photosensitive materials.
Russell, U.S. Pat. No. 3,539,344 relates to coatings of protective material which contain beads of synthetic resins, the coatings being present in the photosensitive material only at surface areas that are not intended for photographic recording. The polymer beads of Russell are for the purpose of providing mechanical separation between stacked film.
Grabhofer et al. U.S. Pat. No. 3,773,976 relates to a transparent roughened protective layer on a silver halide emulsion. The protective layer contains acrylonitrile particles to roughen the surface of a layer to enable it to be written on. There is no teaching that the protective layer prevents oxygen inhibition since the photosensitive layer is a silver halide emulsion layer and not a photopolymerizable layer.
Cook, U.S. Pat. No. 3,240,604 teaches adding polytetrafluoroethylene particles to a layer on a silver halide element to prevent blocking or adhering when the elements are stacked or packaged together. There is no teaching or need to preclude oxygen from inhibiting the light sensitive layer. The layer is designed to be water resistant, not water permeable.
Gray, U.S. Pat. No. 2,731,347 teaches the use of polytetrachloroethylene type polymers for delustering silver halide emulsion layers. It does not disclose acrylic or alkacrylic polymers in an oxygen impermeable protective stratum.